Plants purify poisoned water
Ferns suck up arsenic quickly and cheaply.
By Philip Ball
Water contaminated with arsenic can be cleaned by growing ferns in it, US scientists say.
Mark Elless of the Edenspace Systems Corporation in Dulles, Virginia, and his colleagues have found that a species of fern called Pteris vittata will suck arsenic out of tainted water. The plants reduce the concentration to below the safety limit set by the US Environmental Protection Agency (EPA) in less than a day.
The procedure, called phytofiltration, could provide a cheap way to remove arsenic from water supplies. The ferns would be grown directly in the water, similar to the reed-bed systems currently used to remove organic waste.
"It is an interesting technology," says Andrew Meharg, a specialist in arsenic uptake by plants at the University of Aberdeen, UK. But he thinks it will be best suited to small-scale water cleaning in developed countries, rather than the purification of contaminated water in the developing world.
Arsenic pollution of drinking and irrigation water has emerged as a massive health threat in Bangladesh and India, where wells drilled into aquifers have turned out to be tapping poisoned water.
When the water is used to irrigate rice paddies, arsenic also accumulates in the crop. According to one estimate, 3,000 people may be dying in Bangladesh each year because of arsenic contamination.
Elless and his colleagues hope their ferns could be adapted to help purify water in these countries; the method is potentially very cheap, and the plants grow readily in warm, humid climates like those of south-east Asia.
But Meharg is less optimistic. He points out that the ferns may not be able to cope with the huge volumes of water used for irrigation, and that Bangladesh probably lacks the infrastructure needed to maintain such treatment facilities.
Still, the approach could be valuable in richer countries. For example, thousands of US water-supply systems exceed the new EPA limit for arsenic concentrations in drinking water of 10 millionths of a gram (10 micrograms) per litre.
The limit comes into effect in January 2006; the existing limit is five times higher. For small rural communities, fern filtering of arsenic could be just the thing to achieve this new limit economically.
Pteris vittata was identified three years ago in Nature as a hyperaccumulator of arsenic1. The plants will hold as much as 22 grams of arsenic per kilogram of plant matter, and are hardy and fast growing.
In their latest study, published in Environmental Science and Technology2, Elless and colleagues examined the practical use of their ferns by measuring the arsenic reduction they achieved and the time they took.
The plants reduced an arsenic concentration of 200 micrograms per litre of water by nearly 100-fold, well below the new EPA limit, within 24 hours, the researchers found. The same plants could be used repeatedly with successive batches of tainted water.
Unlike most other arsenic-removal strategies, phytofiltration does not produce an arsenic-rich chemical sludge, of which it is hard to dispose. Instead, squeezing the sap from the plants in presses removes about three-quarters of the arsenic, which can then be extracted for industrial uses.
Ma, L.Q. et al. Nature, 409, 579, (2001).
Huang, J.W., Poynton, C.Y., Kochian, L.V. & Elless, M.P. Environmental Science and Technology, advance online publication (2004).
Nature Science Update, 5/11/04
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